This chapter focuses on how flow in rivers shapes channel geometry, particularly the width and depth of river channels. It first examines the concepts of at-a-station and downstream hydraulic geometry, which relate width, depth, and velocity of flow to discharge. At-a-station hydraulic geometry largely reflects the influence of channel form on flow, whereas downstream hydraulic geometry reflects changes in channel geometry to accommodate downstream increases in discharge. A distinction is made between hydraulic geometry and channel geometry, where channel-geometry relations are not restricted by flow continuity constraints. Classic hydraulic- and channel-geometry analyses are based mainly on statistical analysis of empirical data. By contrast, rational regime theory uses optimization algorithms along with physical reasoning to examine relations among flow, sediment transport, bank stability, and channel geometry. The final section of the chapter explores approaches to channel geometry that consider dynamic change in geometry over time.